As use of and demand for semiconductor devices increases, improvements in size, performance and yield are constantly being developed and improved. Achieving the objectives of miniaturization and higher packing densities continue to drive the semiconductor manufacturing industry toward improving semiconductor processing in every aspect of the fabrication process. For example, at least one and typically more than one photolithography process may be employed during the fabrication of a semiconductor device. Each factor and variable implemented during fabrication should be considered and improved in order to achieve higher packing densities and smaller, more precisely formed semiconductor structures. To increase yield, which is the percentage of finished products leaving the fabrication process compared to the number of products that entered the fabrication process, control and/or quality of individual fabrication processes should be improved through continuous monitoring and/or adjustment.
Semiconductor fabrication is a manufacturing process that includes a large number of steps and/or processes that control and build the devices. The basic processes utilized are layering, doping, heat treating, and pattering. The layering process adds thin layers to a wafer surface. Layers can be insulators, semiconductors and/or conductors, for example, and are grown or deposited through a variety of process including chemical vapor deposition (CVD), evaporation, and sputtering. The doping process adds specific amounts of dopants to the wafer surface, which can cause modification of the layer properties (e.g., change a semiconductor to a conductor). Doping techniques include thermal diffusion and ion implantation. During the heat treating process, a wafer is heated and cooled to achieve specific results. Typically, during heat treatment no additional material is added or removed from the wafer, although contaminates and vapors may evaporate from the wafer. Annealing is a common heat treatment that repairs damage to the crystal structure of a wafer/device, which is generally caused by doping operations. Other heat treating techniques include alloying and driving of solvents.
The pattering process is considered the most important of the four basic processes and is a series of steps designed to remove selected portions of surface layers. After removal, a pattern of the layer is left on the wafer surface. The removed material can be in the form of a hole in the layer or a remaining island of the material, for example. The pattering transfer process is sometimes referred to as photomasking, masking, photolithography, or microlithography. The goal of the pattering process is to create specific shapes with specific dimension (e.g., feature size) as determined by a circuit design and to locate the shapes in the proper location on the wafer surface.
A process control system is generally employed to perform one or more of the pattering or lithography processes. The process control system can control various parameters including development time, resist flow, and the like. The process control system can monitor characteristics including etch rate, dimensions, feature size, etc. to determine whether fabricated devices are acceptable according to design tolerance specifications and control limit(s). Variation of circuit feature critical dimensions, such as transistor gate linewidth can lead to unwanted variation in circuit performance. Reduction of unwanted across-chip transistor performance variation can improve the performance of chips made in subsequent production lots.